A sealed unit, where two panes of glass are fixed and hermetically sealed together or a second 'window' placed inside the original window. Both types of double glazing give heat and noise insulation.
When two pieces of glass are separated by a spacer and sealed or glazed together, it is called double or dual glazing. This dead space creates an insulating barrier to the outside elements. Most modern windows are constructed of double glazed glass. The best windows use a non-metal spacer ( Super Spacer) to further reduce cold transfer from the outside glass to the inside glass.
a sandwich of two separated layers of glass or plastic enclosing air to create an insulating barrier.